The number of Europeans joining Islamist fighters in Syria and Iraq has jumped by a third to around 3,000 in a few months, the EU counter terrorism chief said Tuesday.

In an exclusive interview with AFP, Gilles de Kerchove said the flow of Europeans may have been boosted by the Islamic State militant group's declaration in June of a caliphate straddling Syria and Iraq.

"My own assessment is that we're about 3,000," he said when asked how many European fighters have flocked to the extremists' cause.

In June, he said there were 2,000 such fighters from Europe.

"The flow has not been dried up, and therefore possibly the proclamation of the caliphate has had some impact," de Kerchove said.

His comments came hours after the United States and its Arab allies unleashed deadly strikes from land and sea on jihadists in Syria, opening a new front in the battle against the Islamic State group.

The European fighters, he said, come mainly from France, Britain, Germany, Belgium, The Netherlands, Sweden and Denmark but a few come from Spain, Italy, Ireland and now Austria.

"Even a country like Austria I think has now foreign fighters, which I was not aware of before," he added.

The EU counter-terrorism coordinator nonetheless said he would check his figures with those of European security service chiefs in the next couple of weeks, saying their numbers may be more conservative.

He estimated that between 20 and 30 percent have now returned to their home countries. Some have resumed a normal life while others are suffering from post-traumatic stress disorder. But some have become radicalised and dangerous, he warned.

He said EU countries will prosecute militants if they have evidence they have joined a radical organisation, and will discreetly monitor those for whom they have no evidence.

"The challenge is for each member state to assess each and every returnee, assess their dangerousness and provide the adequate response," de Kerchove said.

He welcomed the expected adoption Wednesday of a UN Security Council resolution designed to make it a criminal offence to go abroad to train for or fight in a holy war.

"The draft (resolution) is really addressing the right questions. But it's of course up to each state and group of states, in our case 28, to go deeper into this," he said.

In some cases, European countries may be more advanced as they have been tackling the problem for longer.

He backed French draft legislation which allows the interior minister to withdraw passports from individuals when there is sufficient intelligence -- rather than evidence -- that they are travelling for jihad.

- 'Lone wolf' risk -

The EU itself boosted security this week after reports that jihadists returning from Syria were planning to target the European Commission building in Brussels.

De Kerchove said much of the reason so many Europeans were able to fight in Syria was because of neighbouring Turkey's proximity to Europe and its role as a mass tourism destination.

On the other hand, Europeans wanting to travel to fight in Afghanistan, Mali, Yemen and Somalia faced geographic obstacles, greater costs and even hotter environments.

Despite the difficulties, the Turks are arresting and sending back many European citizens, he said.

"More needs to be done, that's true. But we work together. And I'm not at all in a blaming game," he said.

"We understand the difficulty of the Turks. We have an excellent dialogue, and we explore together how we can work more closely together," he added.

He said he did not expect well-organised large-scale attacks like those of September 11, 2001.

"But the lone wolf with a Kalashnikov may do a lot of damage," he said.

He expected the risk of retaliatory attacks against Europeans to rise after France joined the United States in carrying out air strikes against jihadis in Iraq.